I. Field of the Invention
The present invention relates to an optical filter element which controls or limits the transmission of light so that only a portion of the total incident light passes through the optical filter element thereby making the element semi-transparent. The element is typically applied or adhered to a light transmissive substrate such as polymer or glass for ultimate use in the field of window manufacturing and particularly in the manufacturing of windows where control of light (e.g., control of absorbed light, reflected light, transmitted light and solar energy rejection) is important.
II. Background Information
Windows are conventionally manufactured with solar control elements or optical filters as a component thereof in order to provide desired coloration and advantageous levels of visible light transmission (VLT), visible light reflectance (VLR), solar energy absorption and total solar energy rejection. Such optical filters are often multi-layered coatings or laminates which are used in combination with a glass sheet or other optical glass device so that the light which passes through the glass also passes through the optical device to produce the desired effects. Typically the optical filter is incorporated in or on the glass or is positioned in close proximity to the glass (such as within the gas containing space between two glass sheets in a double glazed window). The optical filter may be coated or otherwise adhered to a suitable light transmissive substrate such as a polymer in order to produce a structure for combination with the glass. For example such optical filters are conventionally coated onto or otherwise adhered to a polymeric film such as polyethylene terephthalate (PET). Such polymer/optical filter films can be combined with the glass during the manufacturing steps of making a window for residential, automotive or other architectural applications. Alternatively the polymer/optical filter film can be retrofitted onto previously manufactured windows by adhering the film thereto. Such polymer/optical filter films are known as solar control films.
Windows which include an optical filter have characteristic levels of reflection on both sides (i.e., surfaces) of the filter. Thus, such windows which are installed in the walls of a building or vehicle will have a characteristic level of interior visible reflection and exterior visible reflection. Interior visible reflection is the reflection of visible interior incident light (interior light is light which is inside the building or vehicle) while exterior visible reflection is the reflection of visible exterior incident light. (exterior light is light which is outside the building or vehicle). Optical filters which have unequal exterior and interior visible light reflection are asymmetric reflectance (dual-reflectance) filters or films. It is highly desirable to use optical filters wherein the interior visible reflection is less than the exterior visible reflection. Also it is highly desirable to keep reflectance levels to 20% or less.
When optical filters are used for residential or architectural applications such as windows, it is desirable for the filter to have certain properties. Some of these properties include color that does not change hue or shade over time (i.e., color stability); visible light reflectance on both the exterior and interior that is not perceived as xe2x80x9cmirrored xe2x80x9d(20% or less); visually appealing transmitted and reflected color; and the ability to significantly reduce solar heat gain (60% or greater solar energy rejection). Traditionally optical filters for windows (e.g., solar control films) are constructed either from dyed polymer film, clear polymer film coated with a single metal layer or multiple metal layers, or a hybrid structure which includes both dyed film and metallized film. However such traditional optical filters for windows do not provide all of the above desirable properties.
Dyes used in such optical filters have poor color stability and thus optical filters which contain a dyed layer will eventually fade and change color. Also, optical filters which utilize only dyed layers instead of metal layers have inferior solar heat rejection. Films with single or multiple metal layers are perceived as too reflective in the marketplace for films with visible light transmission levels of 35% or less. In other words, the problem with single or multiple layer metal films is that when the metal is thick enough to bring the light transmission to 35% or less, the visible reflectance becomes unacceptably high. The hybrid dyed/metal optical filters described above provide low visible reflectance but are still susceptible to color change and fading problems. In addition when dyed/metal films are constructed with a single dyed layer facing the window, the interior reflectance is excessive.
When optical filters, such as solar control window films, are in contact with a glass window additional thermal stress is induced into the glass. This occurs due to the fact that such filters absorb a portion of the incident solar energy. This absorbed solar energy in turn increases the temperature of the glass for the portion of the glass exposed to sunlight. This increased temperature induces additional thermal stress in the glass structure. If the thermal stress exceeds the tensile strength of the glass thermal stress breakage can occur. Thus it is imperative to produce optical filters and solar control window films which do not produce excessive thermal stress. This is accomplished by minimizing the solar absorption of such filters and films.
To minimize the occurrence of thermal stress breakage, and thus to manufacture commercially viable solar control products, the following limits of solar absorption are recommended by glass, window and solar control film manufacturers as an industrial standard:
Single-pane annealed glass: less than 65% total solar absorption
Dual-pane annealed glass: less than 50% total solar absorption
These absorption rates are measured with the optical filter applied to xe2x85x9xe2x80x3 clear glass. This industrial standard exists as a safeguard against thermally induced breakage. Thus it would be highly desirable to meet this standard while providing the desirable color stability and light altering characteristics. It would also be desirable to meet this standard as described above while also limiting the visible light reflectance on both surfaces of the optical filter so that a window can be produced which has an exterior and interior visible light reflectance of 25% or less.
As noted above one of the parameters which is regulated by optical filters is the amount of solar energy rejection. The term xe2x80x9ctotal solar energy rejectionxe2x80x9d (TSER) is a term of art which describes the percentage of incident solar heat rejected by a glazing system relative to the incident solar radiation. The TSER value equals the solar reflectance plus the portion of the solar absorption that is both re-radiated and conducted/convected to the outdoors. The TSER is expressed as a percentage between 0 and 100%. The higher a window""s TSER the less solar heat is transmits.
Total solar energy rejection may also be expressed in terms of the solar heat gain coefficient (SHGC). The SHGC represents the solar heat gain through the window system relative to the incident solar radiation. SHGC is expressed as a number between 0 and 1. The lower a window""s SHGC, the less solar heat is transmits. The sum of TSER (in decimal form) and SHGC value is 1. Thus if the TSER of a specified optical filter is 65% then the SHGC is 1 minus 0.65 which equals 0.35.
It is an objective of the present invention to provide an optical filter for a window or the like which has the desirable characteristics as outlined above. In particular it is an objective of the invention to produce a colored optical. filter with color stability without the use of dyes.
It is another objective of this invention to produce a color stable optical filter that provides increased or high levels of total solar energy rejection with reduced interior and exterior visible reflectance as compared to single or multiple layer metal films.
It is a further objective of this invention to provide a color stable optical filter with reduced interior and exterior visible reflectance as compared to existing commercially available all-metal asymmetric reflectance (dual-reflectance) optical filters.
It is also an objective of this invention to accomplish the above objectives for dark optical filters (where VLT is 5% to 25%) while maintaining total solar energy rejection at a level of at least 60% and maintaining exterior and interior visible reflectance at 20% or below.
It is also an objective of the present invention to accomplish one or more of the above objectives in an optical filter in which the interior visible light reflectance is less than the exterior visible light reflectance.
It is also an objective of the present invention to produce a neutral colored optical filter which has neutral transmitted color and neutral reflected color from both the exterior and the interior.
These and other objectives which will become apparent in the disclosure of the invention are accomplished by combining an interference Fabry-Perot interference structure with a massive Fabry-Perot structure in a single optical filter. The interference Fabry-Perot structure is a multi-layered stack containing the layers: metal/interfering dielectric/metal. The massive Fabry-Perot structure is a multi-layered stack containing the layers: metal/massive dielectric/metal.
The optical filter which contains the combination of an interference Fabry-Perot structure and a massive Fabry-Perot structure is advantageously adhered to a suitable substrate of the type which is conventionally used in solar control films, except, as noted above, no dye is required in the film. For example the metal/interfering dielectric/metal layers may be sequentially coated onto the substrate to form the structure xe2x80x9csubstrate/first metal/interfering dielectric/second metalxe2x80x9d. Conventional coating techniques such as sputtering and/or vacuum evaporation may be used to sequentially apply the coatings onto the substrate.
The massive Fabry-Perot structure is applied directly onto the interference Fabry-Perot structure. Advantageously one of the metal layers of the interference structure is used as a metal layer in the massive Fabry-Perot structure. In other words, one metal layer in the optical filter is shared by both the interference Fabry-Perot stack and the massive Fabry-Perot stack. Such an optical filter adhered to a substrate will have the layers: substrate/metal/interfering dielectric/metal/massive dielectric/metal. The metal between the interfering dielectric and the massive dielectric is shared by both the interference Fabry-Perot structure and the massive Fabry-Perot structure so that the interfering Fabry-Perot structure in the optical filter has the layers: metal/interfering dielectric/metal and the massive Fabry-Perot structure has the layers: metal/massive dielectric/metal although the entire optical filter requires only three metal layers therein.
One advantage of the above structure which shares a metal layer between the two Fabry-Perot structures is that the unshared metal of the massive dielectric can be directly laminated onto the interfering Fabry-Perot structure through the use of an adhesive which also functions as the massive dielectric layer. Adhesives which can function in this manner are dielectric adhesives. The ability to apply the massive Fabry-Perot structure to the interfering Fabry-Perot stack is advantageous because lamination is easier and less costly than coating methodologies such as sputtering.
At least one of the three metal layers is an absorbing metal layer (Ma) and at least one of the three metal layers is an infrared reflecting layer (Mir). The third metal may be either an absorbing metal layer or an infrared reflecting layer. Such films have desirable visible light reflection and visible light transmission characteristics. Preferably the stack has the structure: Ma/interfering dielectric/Ma/massive dielectric/Mir.
Either side of the stack may be adhered to the substrate. Thus there are two possible structures for the above preferred stack which includes the substrate. These are: substrate/Ma/interfering dielectric/Ma/massive dielectric/Mir and the structure Ma/interfering. dielectric/Ma/massive dielectric/Mir/substrate. In these structures the interfering Fabry-Perot structure has the layers Ma/interfering dielectric/Maand the massive Fabry-Perot structure has the layers Ma/massive dielectric/Mir.
Thermal stress and undesirable reflectance are minimized by employing layers of unequal thickness (i.e., asymmetric structural design) in the interference Fabry-Perot structure and, in particular, thermal stress and undesirable reflectance are minimized advantageously by employing two absorbing metal layers of unequal thickness in the interference Fabry-Perot portion of the optical filter. The thickness ratio of the thicker absorbing metal layer to the thinner absorbing metal layer is greatest in dark optical filters and the ratio is diminished to approximately 1:1 in an optical filter which has a visible light transmission of 35% or more. To produce a 35% VLT design, the two absorbing metal layers are extremely thin such that the thinness approaches the limits on how thin these particular metal layers can be manufactured. Accordingly, there are limits as to the maximum VLT for the optical filters which are manufactured in accordance with this invention. For practical purposes, the filters of this invention will have a maximum VLT value of approximately 50%.
Suitable substrate materials include PET and glass. Both the substrate and the multi-layered structure of the optical filter may be covered with conventional materials which are known in this field of technology. For example, polyvinyl butyrate (PVB) or an adhesive may be used to cover the substrate. Suitable materials for covering the multi-layered structure on the side opposite the substrate include PET, PVB, hardcoats and adhesives. PVB is conventionally used in safety glass laminates.
Fenestration constructions which may include the optical filter of this invention include the following:
1. Glass in a PVB laminate (glass substrate/optical filter stack/PVB/glass)
2. Film in a PVB laminate (glass/PVB/film substrate such as polymer/optical filter stack/PVB)
3. Film suspended in an insulated glass unit (glass/air-space/substrate film/optical filter stack/air-space/glass)
4. Film adhered to a glass surface (e.g., standard retrofit solar control film or low-e retrofit solar control film).
Although metal layers are used to make the optical filter of this invention, various nonmetallic elements and metal compounds as further described herein may be used in place of the metals.
Reference is made herein to neutral color both with respect to transmitted color and reflected color. The colors described as being suitable in this invention, and in particular, the neutral transmitted color and neutral reflected colors established in this invention are measured by the color measurement specification established by the Commission International de L""Eclairage. This method for color measurement measures the quantities L*, a* and b*. The relevant parameters for the color used in the present invention are the values a* and b* in the CIE L*, a*, b* method for color measurement.
The transmitted color which is desired for the optical filter of the present invention has an a*value of xe2x88x923 to +1 and a b* value of xe2x88x926 to 2. The reflected color (interior or exterior) for the optical filters of this invention has an a* value of xe2x88x924 to 0.5 and a b* value of xe2x88x924 to 6.
Reference is made herein to dark, medium and light optical filters. Dark filters have visible light transmissions of 5%-25%. Medium filters have visible light transmissions of 26%-45%. Light filters have visible light transmission of more than 45%.